Study On Plasma Wave Instability And Negative Differential Resistance Effect Of Heterojunction Field Effect Transistor

With the rapid development of terahertz(THz)radiation source and detector,THz technology has gradually been used in imaging,wireless communication,medical detection and other fields with many advantages,and it has become the focus of researchers.In the current application,the development of THz detectors is relatively rapid,but the demand for small-sized and coherent solid-state THz radiation sources is very urgent.Heterojunction field-effect transistors(FET)can be used for THz emission and detection under the mode of plasma wave excitation and the frequency is tunable,which has a good prospect in the future development of terahertz technology.So,it received a lot of attention.The theory of solid-state emitter devices based on the plasma wave mechanism has been well proposed,and the experimental results obtained on various device structures show a good agreement with the theoretical analysis.However,there are some problems encountered in the research of plasma wave devices,including how to increase the transmission power density of THz,the band-spreading mechanism in the emission spectrum,and the negative differential resistance(NDR)effect has no uniform mechanism explanation.One of the important reasons is that the existing simplified models for characterizing plasma wave devices ignore the thermal effects of the device(the energy equation is not considered),which makes the simulation results deviate from the experimental results.Aiming at this problem,this thesis mainly focuses on the theoretical analysis and numerical simulation of plasma wave instability and corresponding NDR effect of MESFET and HEMT device structures.The main work includes the following four aspects.First,the device simulation platform for 2D hydrodynamic model of FET devices working at ultra-high frequency is improved.The platform uses finite-difference time-domain(FDTD)method and 2D hydrodynamic model for numerical simulation.Using a flexible and configurable method to introduce the Ga As band structure into the model with electron energy dependent mobility,relaxation time,and effective mass parameters.It is possible to carry out simulations of device structures of different materials.The simulation function is verified in the short-channel Ga As-based MESFET device structure.Second,the numerical simulation of plasma wave instability was carried out based on the 2D device simulation platform.The numerical simulation results of the electron density instability in the channel are obtained through the full hydrodynamic model and the setting of the 2D asymmetry boundary conditions.The physical process of plasma wave selfexcitation was characterized.The transient process of the average energy of the electrons is described,and the numerical simulation results are given.Analysis shows that there is ramp wave propagation in the channel,and the output characteristics have a significant NDR effect.Then,the two-temperature model under non-equilibrium conditions was used to theoretically analyze the possible NDR effects.The NDR effect accompanying the THz emission in the output characteristics of HEMT devices reported in the literature is analyzed and studied.Because of its low threshold voltage and large peak-to-valley ratio,it is different from the traditional electronic valley-to-valley transfer.Adjust the model parameters according to the material characteristics and the existing GaN HEMT experiments.The analytical calculation results are fitted with experimental results with good consistency.Finally,the band characteristics of Al GaN/GaN-based recessed gate HEMT devices are studied and the possibility of NDR effect is analyzed.It is found that there is an NDR threshold electric field,and the NDR threshold voltage has a corresponding relationship with the difference between the sub-levels of the heterojunction quantum wells.Using TCAD software simulation,a possible NDR generation mechanism caused by real space transfer(RST)of resonant tunneling is proposed.The enhanced device is obtained by changing the threshold voltage through the recessed gate structure.It has stable DC output characteristics and simulation results of the gate tunneling current.